Back in the old days, Halloween costumes were worn so ghosts and spirits would think you were 'one of them' and not bother trying to haunt you. If you think that's clever, then bear in mind that without the best Halloween costume, the same spirits and ghosts would have likely brought you back with them to the underworld in a hunt for fresh meat. So basically, the saying about necessity being the mother of invention comes to play hugely when talking about the first Halloween costume ideas.
